This invention relates to the preparation of branched polycarbonates and to reagents suitable for use therein.
Branched polycarbonates are becoming of increasing importance for many purposes, especially in blow molding operations as exemplified by the fabrication of water bottles. Their non-Newtonian properties are extremely important under blow molding conditions. Branched polycarbonates useful for this purpose are typically prepared by the incorporation of a hydroxyaromatic compound having more than two hydroxy groups into a conventional polycarbonate-forming reaction mixture.
Thus, U.S. Reissue Pat. No. 27,682 describes the preparation of branched polycarbonates in a conventional interfacial reaction or from chloroformates. U.S. Pat. No. 4,415,725 describes a similar method which may employ a carbonyl halide such as phosgene (as in the interfacial procedure), a haloformate or a diarylcarbonate. U.S. Pat. No. 5,021,521 describes the preparation of branched polycarbonates by reactive extrusion of a linear or branched polycarbonate with a branching agent of the type described above.
Many of the branching agents considered useful for the preparation of branched polycarbonates are trisphenols. The most common of these is 1,1,1-tris(4-hydroxyphenyl) ethane. Others are disclosed in the aforementioned patents with particular reference to Re Pat. No. 27,682.
It has been found, however, that the branched polycarbonates obtained from trisphenols in a reactive extrusion procedure do not have optimum viscosity characteristics for blow molding. The key properties for this purpose are a relatively low viscosity during high shear melt mixing such as extrusion, and a high viscosity under low shear conditions such as those encountered by a parison before and during blow molding.
The melt strengths and melt viscosities of trisphenol-branched polycarbonates prepared by equilibration are too low to permit their fabrication into parisons. Only by the employment of very high molecular weight polycarbonates, such as those having weight average molecular weights greater than 175,000 relative to polystyrene, is it possible to produce branched products which even approach the desired blow molding properties in viscosity characteristics at low shear.
It has been thought that the use of tetraphenols rather than trisphenols in reactive extrusion processes might improve the viscosity characteristics of the branched products. However, many of the tetraphenols disclosed as useful for this purpose in the art previously identified have serious disadvantages. These include the presence of benzylic hydrogen atoms, which decrease their thermal and oxidative stability; the presence of ortho-substitution, which decreases reactivity; and a characteristic bright color due to a high degree of conjugation, which carries over into the polymeric product and makes it unattractive for use.
The search continues, therefore, for tetraphenolic branching agents which afford polycarbonates with the desired viscosity characteristics and which do not suffer from other serious deficiencies.